Nowadays, switch mode power supply (“SMPS”) is becoming widely used for electronic devices. The semiconductor switch in the SMPS is turned on and turned off periodically to supply energy to a load with switching frequency generally over ten KHz. SMPS can be smaller, lighter and more efficient compared to a conventional linear power supply.
There are many circuit topologies for SMPS, such as buck, boost for the non-isolated type and half-bridge, flyback for the isolated type. Taking a SMPS based on flyback topology as an example, it has the following functional modules: an energy input module, an energy couple module, an energy output module, a feedback module and a controlling module. An alternative current (“AC”) voltage is input through the energy input module and transformed into a smooth direct current (“DC”) voltage, which is further transformed into a high frequency signal through periodically turning on and turning off the switch based on a feedback signal. The high frequency signal is coupled by a transformer and a steady DC voltage is output through the output module.
Besides normal operating mode, electronic devices can also operate under a reduced load or standby mode (generally referred to as “light load”). Under light load conditions, the load needs very low power output from the power supply. If the controlling module still uses the original frequency to drive the switch, the loss on the switch becomes significant and the efficiency becomes low. One solution is decreasing the frequency to a lower level. However, from an efficiency point of view, the light load condition requires switch frequencies below 20 KHz. Thus, the frequency is in the audible frequency range to cause noise issues.
Another solution is adopting a burst-mode function, which alternately modulates the switch based on a high frequency signal within a period of time (labeled with Mon), and in another period of time (labeled with Moff) the switch is kept off. Thus, the equivalent switching frequency is decreased to reduce the switching loss. However, the period of time Mon and Moff is typically automatically adjusted according to the power supplied to the load, which may cause the value of Mon and Moff to be uncertain, resulting in noise issues.